This invention relates to a novel photographic method for preparing a screen structure for a cathode-ray tube (CRT) and particularly, but not exclusively, to a novel method for preparing a light-absorbing matrix for a color television picture tube.
Color television picture tubes which include a light-absorbing matrix as a structural part of the luminescent screen have been described previously; for example, in U.S. Pat. Nos. 2,842,697 to F. J. Bingley, and 3,146,368 to J. P. Fiore et al. These patents describe color television picture tubes of the aperture-mask type (also called shadowmask type) in which a light-absorbing matrix is located on the inner surface of the faceplate of the tube. In this structure, the matrix has openings therein which are filled with phosphor material.
A reverse-printing method for preparing a light-absorbing matrix for a CRT is described in U.S. Pat. No. 3,558,310 to E. E. Mayaud. In a preferred embodiment of that method, the inner surface of the faceplate of a CRT is coated with a film of water-based photopolymeric material, typically a dichromate-sensitized polyvinyl alcohol. A light image is projected on the film to insolubilize selected regions of the film. The film is developed by flushing the film with water to remove the still-soluble regions of the film while retaining the insolubilized regions in place. Then, the developed film or stencil and the bare areas of the surface are overcoated with a layer containing particles of light-absorbing material, such as graphite. Finally, the overcoating is developed by removing the stencil together with the overlying overcoating, while retaining the overcoating on the areas of the surface not covered by the stencil.
Such a process produces satisfactory light-absorbing matrices in automated and semiautomated factory production. However, frequently, the overall appearances of matrices are marred by streaks and swirls. Such streaks and swirls are believed to result from the leaching of soluble and/or partially-soluble material from the stencil, which leached material is then redistributed in a random manner by gravitational and centrifugal forces during and after development of the film. Careful examination has shown that these streaks and swirls are merely cosmetic, resulting from very slight nonuniformities in the sizes of the openings in the matrix. Although they may have a trivial effect on the performance of the tube, nevertheless streaks and swirls adversely affect the salability of picture tubes which have them.
Polyvinyl alcohol as used by the CRT maker is a product of polyvinyl-acetate manufacturing whose main uses are in the paint and adhesive industry. Attempts have been made to supply a special electronic-grade of polyvinyl alcohol, but the cost is prohibitive because the total amount used is small. Therefore, the CRT maker uses materials made for the textile, paper and binder industries. The chemical character of those polyvinyl alcohols is dictated by the uses in the aforementioned industries.
The principal factors governing the properties of polyvinyl alcohol are the degree of polymerization and the percent hydrolysis. While proper blending of batch-produced polyvinyl alcohol can provide products within a given saponification range, such materials will contain much larger amounts of high and low molecular weight fractions and of portions varying widely in degrees of hydrolysis. This type of manufacturing variation produces products with varying degrees of cold-water solubility. The CRT maker, to make matrix stencils, dissolves away that part of the polyvinyl alcohol not insolubilized by the actinic radiation, while retaining the insolubilized part as a stencil. All regions of the stencil have some partially-soluble material therein. This partial solubility of the polyvinyl alcohol varies with the lots supplied by the manufacturer.
Prior to coating the photopolymeric film on the faceplate surface, the dry, powdered polyvinyl alcohol is dissolved in water. This step also may introduce variations in the cold-water solubility of the stencil-making material.